1. Field of the Invention
The present invention relates to an optical connector with optical fibers, and in particular it relates to an optical connector with optical fibers in which small diameter optical fibers are connected.
2. Description of Related Art
Single fiber optical connectors and multi-fiber optical connectors are used in optical interconnection techniques. As a multi-fiber optical connector, an MPO type optical connector is widely used (for example, refer to Japanese Unexamined Utility Model Application, First Publication No. S62-104203).
An optical fiber to be inserted in the optical fiber opening of a ferrule of an optical connector is a bare optical fiber in which a resin coating layer is removed from a distal end thereof.
As is known, in an MPO type optical connector, a PC connection (PC: physical contact) is used in which end faces of the optical fibers are butt-coupled to prevent Fresnel reflection due to an air layer.
In response to a requirement for super multiple fibers in the field of optical interconnection, an MPO type optical connector has been developed in which small diameter optical fibers with resin coating layers of 125 μm diameter are inserted into optical fiber openings in a state in which the resin coating layers remain attached (refer to September 2007, Institute of Electronics, Information and Communication Engineers “Super Multi-fiber MPO Connector Using Optical Fibers For Optical Interconnection, Authors: 1 Ishikawa, Kunihiko Fujiwara, Yukio Hayashi”).
The small diameter optical fiber used therein has a construction in which a bare optical fiber with a cladding outer diameter of 80 μm is provided with a first coating layer with a high Young's modulus and a colored coating layer (colored layer) with an outer diameter of 125 μm.
An optical connector in which optical fibers on which the resin coating layers remain, are inserted into the optical fiber openings is disclosed for example in Japanese Unexamined Patent Application, First Publication No. 2003-322760 and Japanese Unexamined Patent Application, First Publication No. 2004-258193.
The MPO type optical connector is ground in a convex shape such that the optical fibers protrude from the connecting end face of the ferrule consistently.
However, it is known that the convex grinding process is complicated, and the process control is difficult, so grinding deficiencies, such as an insufficiency of the protruding amount (protruding length) of the optical fibers, dispersion in the protruding amount, angular deficiency of the ferrule end face, and the like, are likely to occur.
In particular in a super multi-fiber optical connector, in which multiple rows of optical fiber openings are arranged in two directions, the area of the connecting end face is large compared with the case where the optical fiber openings are arranged in one row, and hence there is a problem in that grinding defects are likely to occur.
On the other hand, dispersion in the protruding amount of the optical fibers due to grinding defects can be corrected by increasing the connection pressure when the optical connector is connected, in order to equalize the protruding amount.
However, the pressing force (stored spring pressure) of a standard MPO type optical connector is specified to be within a range in which the optical connector will not be damaged. Therefore, in the case where the dispersion in the protruding amount is large, it is difficult to compensate (equalize) completely.
Therefore, in order to compensate for large dispersion in the protruding amount, an optical connector is proposed in which optical fibers are attached to a ferrule without removing their resin coating layers, so that the resin coating layers of the optical fibers serve as a cushioning material against the pressing force (contact force) applied when connecting.
This construction makes it easy for the optical fibers to move toward the optical fiber openings when the pressing force is applied to the ends of the optical fibers, so that optical fibers that protrude excessively are pushed toward the optical fiber openings.
In this manner, due to the large connecting end face and the large number of optical fibers to be connected, even in the case where dispersion occurs in the protruding amounts of the optical fibers, it is possible to optimize the protruding amounts of all optical fibers to realize excellent PC connections.
However, it is proven that if the Young's modulus of the resin coating layer is reduced in order to compensate for great dispersion, conversely the optical connection characteristics deteriorate.
That is, it is proven that if the Young's modulus of the resin coating layer is too high, the resin coating layer loses flexibility, so that its buffering function deteriorates, but conversely, if the Young's modulus of the resin coating layer is too low, the core locations of the ends of the optical fibers are unstable, so that the optical connection characteristics deteriorate.
On the other hand, in a super multi-fiber optical connector, it is required to increase the number of optical fibers to be connected while maintaining excellent connection characteristics.
However, if a ferrule for an MT optical connector is 6.4 mm wide and 2.5 mm thick, for example, the number of optical fiber openings, which is limited by the width, is approximately 12 fibers in practice.
A ferrule of an optical connector type named MINI-MT, conforming to JIS C5984, is 4.4 mm wide and 2.5 mm thick, and since the number of optical fiber openings is limited by the width, it is approximately 4 fibers in practice. In this manner, in a ferrule with a standard shape, the width and thickness of the connecting end face are limited, and there is a limitation to the increase in the number of optical fiber openings. Therefore, it is not possible to significantly increase the number of optical fibers to be connected.
Furthermore, since the internal cavity (adhesive filled cavity) is small, if a large number of optical fibers are inserted forcibly, the optical fibers will be damaged, or optical loss will occur. Therefore, there is a limitation to the number of optical fibers to be connected.
On the other hand, for the optical connector housings for housing ferrules, since internationally there are de facto standards and de jure standards (for example, MPO connector: conformed to JIS C5980), it is difficult to increase the size of the ferrule.
Moreover, in the case where the size of the ferrule is increased, in an application in which a plurality of ferrules is mounted on an optical wiring substrate at a high density, there is a problem in that the packing density of the ferrules drops.
Owing to the above reasons, it is not easy to increase the number of optical fibers to be connected to the ferrule.
Therefore, an optical connector is desired in which the width and thickness of the connecting end face of a ferrule are not excessively larger than those of the standard shape, or which can prevent them getting too large, and in which the number of optical fibers that can be connected is large while having excellent optical connection characteristics.